Frozen Earth: The Once and Future Story of Ice Ages (3 page)

Did plate tectonics and the movement of continents play a role in the Earth’s earlier ice ages?
We don’t know for sure, because as scientists probe further and further back into our planet’s history the evidence becomes increasingly fragmentary.
The question of timing is crucial: for example, was the onset of an ancient ice age coincident with a continent-to-continent collision like the one that raised up the Himalayas, or not?
Dating events accurately enough to answer such questions is more easily said than done.
But one thing is clear from recent research: greenhouse gases, particularly carbon dioxide, played a major part in the initiation and the cessation of past ice ages, just as they have for the Pleistocene Ice Age.

Take, for example, the “Snowball Earth” theory, described in chapter 8.
Over the past several years evidence has continued to accumulate that severe glaciation, with permanent glaciers on the continents and ice covering even tropical seas, occurred during several discrete ice ages between about 600 and 750 million years ago.
One of the problems many scientists initially had with the concept of a completely frozen Earth was that it would have been very difficult to melt: an ice-covered planet would reflect so much of the sun’s energy that it would stay frozen.
However, under such conditions it is likely that enough carbon dioxide (from volcanic eruptions) would eventually accumulate in the atmosphere to produce a “super greenhouse” world, leading to collapse of the ice sheets.
Ending Snowball Earth–like glaciations may not have been as difficult as once thought.
But what initiated these extreme events?

Since the first publication of this book, computer models of global climate have become ever more sophisticated, capable of incorporating more, and more varied, factors that influence climate.
Several groups of scientists have used these models to investigate the probable forcing
factors most important for initiating Snowball Earth–like conditions.

At the time of Snowball Earth glaciation, the planet was a very different place than it is today.
For starters, the surface received about 6 percent less solar radiation (this is well known from studies of how stars like our sun evolve).
Furthermore, all the evidence points to low-latitude locations for most of the existing continents, with none at the poles.
Both of these boundary conditions are important for understanding the Snowball Earth glaciations.

The computer models don’t tell us exactly what happened, and different versions give slightly different results.
But all of the simulations point to the importance of two primary climate forcings: the reflectivity (albedo) of sea ice, and the amount of carbon dioxide in the atmosphere.
Even with solar radiation only 94 percent as strong as it is today, very low greenhouse gas concentrations are crucial for initiating Snowball Earth episodes in all climate models because—with no continents in polar regions—extremely low temperatures are necessary to initiate freezing of the high-latitude seas and maintain year-round ice cover.
As cooling proceeds under low greenhouse gas conditions and ice cover expands, however, albedo becomes the dominant factor and eventually results in runaway cooling.
Exactly how much of the planet must be covered with ice and snow for this to happen varies depending on the model used.
But the point at which runaway cooling begins can’t be reached at all without very low greenhouse gas concentrations.

What lessons do the climate models have for the Anthropocene (an informal but very useful label for the time in our planet’s history when human activity has overtaken natural processes as a primary driver of atmospheric chemistry and other aspects of our environment)?
One startling conclusion from the best and most recent models is that even after anthropogenic carbon dioxide emissions slow down or stop, their effects will persist for much longer than is generally realized: tens of thousands of years.
As the science journalist Mason Inman put it, “carbon is forever” (and he wasn’t referring to diamonds, which are pure carbon).

Why do the effects of greenhouse gas emissions last so long?
Won’t the Earth start to cool down when humans stop putting greenhouse gases into the atmosphere?
The simple answer to the first of these questions is that the climate system is complex and takes a long time to approach a new equilibrium state; the answer to the second is yes, but slowly and only (for thousands or perhaps even tens of thousands of years) to temperatures well above those of the period before the emissions began.

Throughout the glacial-interglacial cycles of the Pleistocene Ice Age, carbon dioxide in the atmosphere has fluctuated between a low near 170 parts per million during the coldest intervals to about 300 ppm during the warmest.
Today it stands near 395 ppm, the high value mainly due to the burning of fossil fuels.
Even taking into account pledged emission reductions, the concentration is expected to continue rising and will likely exceed 850 ppm by the end of the twenty-first century.
If carbon emissions were to miraculously fall to zero then, which appears less and less likely with each passing year, climate models indicate that atmospheric carbon dioxide would still be close to 500 ppm a thousand years later.
Global average temperatures would still be several degrees Celsius (more than 5°F) higher than those of today.
If we end up burning all of the Earth’s fossil fuel reserves, atmospheric carbon dioxide will rise even higher over the next few centuries, to levels approaching 2,000 ppm, and recovery to conditions resembling those of today will take correspondingly longer—hundreds of thousands of years.
Although about half of the anthropogenic carbon dioxide will eventually dissolve in the ocean, and chemical weathering of surface rocks will gradually consume most of the rest, these are slow processes.
The atmospheric content—and the Earth’s surface temperatures—will remain high for a very long time.

In the absence of human activity, the cycles of glacial and interglacial periods that characterize the Pleistocene Ice Age would continue, paced by Northern Hemisphere insolation changes.
The next severe glaciation would occur some fifty thousand years from now, when the Earth’s
orbital parameters will result in low summer insolation at high northern latitudes.
Once again ice would advance over large swaths of North America, northern Europe, and Asia.
But if human activity releases so much carbon dioxide into the atmosphere that greenhouse warming overwhelms the cooling effect of decreased insolation, there will be no Northern Hemisphere glacial advance in fifty thousand years.
The next glacial period will not occur for at least another half a million years, by which time most anthropogenic carbon dioxide will be gone.
It is astonishing to realize that human activity over just a few centuries could have such a profound effect on our planet, stretching tens to hundreds of thousands of years into the future.

To put things in perspective, I should point out that the Earth has experienced periods in the past—even very long periods—with atmospheric carbon dioxide of several thousand ppm, high global average temperatures, and no permanent glaciers except perhaps for a few small high-altitude ice fields.
However, that was long before humans arrived on the scene and existing life had adapted to conditions we would consider extreme.
The greenhouse gas content of the atmosphere is now rising at a rate unprecedented in the Earth’s long history, entirely because of human activity.
Most of the consequent environmental changes will occur over the next few centuries.
Unless geoengineering solutions can be found—large-scale projects designed to slow or stop global warming by a variety of methods, including extracting carbon dioxide from the atmosphere and storing it permanently—humankind will have to adapt very nimbly in order to avoid the partial or wholesale collapse of nations and societies.
The environmental changes, including higher global temperatures, higher sea level, and potentially drastic changes in biological diversity and species distribution, will affect agriculture, human health, and all populations living close to sea level.
Who would have thought that studies of ice ages could give us such insight?

Doug Macdougall

October 2012

CHAPTER ONE

Ice, Ice Ages, and Our Planet’s Climate History

The American author and historical popularizer Will Durant once wrote, “Civilization exists by geological consent, subject to change without notice.”
That is not a new idea, even if Durant phrased it especially well, but nowadays many historians scoff at the notion of environmental determinism, the possibility that climate or geology may have seriously affected the course of human history.
And yet there are still many places on this planet where Durant’s observation rings true, especially places with extremes of climate.
One such is the arctic regions, particularly Greenland.
Ninety-five percent of that island country is covered by ice.
Towns and villages cling to the coastline; at their backs loom glaciers a thousand meters thick: gleaming, white, blue, clear, transparent ice.
The icecap weighs on the land like a lead brick on a floating plank, pressing it down below the level of the surrounding sea.
If the ice were suddenly removed, the waters of the ocean would rush in to take its place.
The glaciers seem fixed and static, but in reality they are dynamic, in constant slow movement outward from their thick centers.
New snowfall adds to their mass every year, but at the margins they calve off apartment-block-sized chunks of themselves and send flotillas of weirdly shaped icebergs sizzling and crackling and sometimes eerily and silently floating down the fjords to the sea.
The icebergs carry
pieces of Greenland with them too, sand, pebbles, and boulders gouged and scraped from the land, later to be dropped far out at sea as the ice melts.
The Inuit of Greenland have lived with the ice of glaciers for thousands of years.
They are truly people of the ice age.
Most of the rest of us have been affected by the ice age too, but in less obvious ways.

Permanent icefields—that is, large glaciers—are not common in mainland North America.
In the mountainous west, in Alaska and in the Yukon, there are small high-altitude glaciers, but in the overall scheme of things, they are fairly minor features of the landscape.
However, as a boy, like many others both in North America and northern Europe, I grew up surrounded by the work of ice.
Like most others, I was, at the time, completely unaware of that fact.
I am not referring to the ice of a skating rink or of a January puddle.
Rather, this was ice just like that of Greenland today, or of Antarctica, ice of vast extent and kilometers thick that blanketed huge swathes of the Northern Hemisphere thousands of years ago.
It reached down from centers in Canada and Scandinavia and covered the sites of cities such as Boston, Detroit, and Hamburg.
Its legacy is everywhere even today, from the geography of our waterways to the distribution of native peoples in the New World.
It ground up solid rock to make the sand of countless beaches and the soil of midwestern farms in the United States.
It sculpted rolling hills and long valleys across the landscape.
It scraped up soil and rocks as it flowed, and dumped the debris as terminal moraines in places like Cape Cod and Long Island, New York, far from its original home.
It even picked up diamonds from still-undiscovered deposits in Canada and transported them to the United States, twenty thousand years before NAFTA was conceived.

The present-day ice sheets of Greenland, and the glaciers in Alaska and arctic Canada, are residual from that once much more extensive ice covering of the Northern Hemisphere.
But it was only in the nineteenth century that the existence of those great ice sheets of the past began to be recognized.
Although some of our distant ancestors lived cheek by jowl with the gigantic ice caps, the small glaciers that still
survived in high mountain regions by the dawn of modern civilization gave few clues to the earlier extent of ice.
The massive ice sheets of Greenland and the Antarctic were far from the consciousness of most of the world’s population and remained largely unexplored until the late nineteenth and early twentieth centuries.
Except for a few small mountain glaciers in Switzerland, there were no glaciers close to the centers of learning that could serve as examples.
The story of the ice ages had to be worked out from other, much more tenuous, evidence.
Like most other scientific advances, the realization that the Earth has periodically been gripped in ice ages didn’t come in a single Eureka!
moment.
Rather, it developed over a period of time and through the efforts of many naturalists and other close observers of the natural landscape.
It came at a time when the science of geology was still young, when the concept that the Earth had an almost inconceivably long history was still controversial, and when the practice of making careful and systematic observations of the natural world was still relatively novel.
The ice age had left its marks abundantly on the lands of the Northern Hemisphere.
The signs were familiar to farmers and travelers, but for the most part their origins were obscure.
It took keen observation, insight, and imagination to recognize in these marks the events that they actually record.
And in spite of the fact that by the early part of the nineteenth century many scientists had discarded the notion that nearly all features of the landscape resulted from the biblical Flood, such ideas died hard.
Some theologians and others prominent in society thundered “blasphemy” at the idea of an ice age.
Even if they didn’t have strictly theological objections, when the idea that northern Europe had once been buried beneath a huge glacier was first proposed, many contemporary scientists summarily dismissed it.
There was no analog.
They could not conceive of such a drastic transformation of the countryside where they now saw only farmland, forests, and rural villages.
From the perspective of a single human lifespan, or even on the timescale of a few generations, the Earth appeared to be quite an unchanging place.

In hindsight, it is easy to say that the geological evidence for ice ages was overwhelming and to wonder why such periods in the Earth’s past were not recognized earlier.
And to be fair, even in the eighteenth century, nearly a hundred years before the term “ice age” was coined, there were already a few bold scientists who had begun to recognize the significance of the evidence.
They and others who studied the Earth by careful observation were gradually eroding the influence of theologians who tried to shoehorn virtually every observation of the natural world into a literal biblical framework.
Still, widespread debate about the reality of ice ages only began in earnest in the 1830s.
The very first use of the term, as far as is known, was in a short, humorous poem written by a German botanist named Karl Schimper, who read and distributed copies of his little literary contribution to friends and colleagues at a scientific gathering in Switzerland in February 1837.
Schimper was a brilliant but delusional scientist who was eventually committed to an asylum, where he died in 1867.
He never became a formal participant in the debate about ice ages, nor did he produce any published works on the subject, but he was a close friend and colleague of the forceful and charismatic Swiss naturalist Louis Agassiz, who today is the person most closely associated with the formulation of ideas about a global ice age.
Significantly, Agassiz was brought up literally in the shadows of the Alps, and glaciers—small mountain glaciers to be sure, but glaciers nevertheless—were part of the natural landscape of his childhood.
By all accounts, Agassiz, a biologist whose first love was fossil fish, was a vigorous, highly intelligent, and very observant scientist.
Like most of his contemporaries, he was initially skeptical about the claim that Alpine glaciers had been much more extensive in the past.
But his conversion was rapid when he realized that many of the same landscape features that he observed being produced by contemporaneous mountain glaciers were also present far afield, in the ice-free valleys of his native country and even far beyond.
Rural folk who encountered such features in their daily lives had reached a similar conclusion much earlier than Agassiz.
The only way they could explain the large and exotic boulders
they sometimes found plopped down in their fields was that they had been carried there by ice.
That meant that in the past the glaciers must have extended far beyond their current boundaries.